The objective of this study is to compare the efficacy and safety of CGS-16949A vs megestrol acetate in post-menopausal women with estrogen-receptor positive and/or progesterone-receptor positive breast cancer (or estrogen-receptor unknown/progesterone-receptor unknown) who relapse while receiving adjuvant tamoxifen therapy. Patients must have had surgical therapy for their primary tumor plus or minus adjuvant chemotherapy and plus or minus adjuvant radiotherapy in the immediate time period after the surgery.